The present invention relates in general to power switching devices in an inverter bridge, and, more specifically, to inverter drive systems for electrified vehicles using discrete power switching circuits having structures to increase common source inductances.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system as shown in FIG. 14 may include a DC power source (such as a battery pack or a fuel cell) 140 coupled by contactor switches to a variable voltage converter (VVC) 141 to regulate a main bus voltage across a main DC linking capacitor 142. An inverter 143 is connected between the main buses and a traction motor 144 in order to convert the DC bus power to an AC voltage that is coupled to the windings of the motor to propel the vehicle.
The inverter includes transistor switching devices (such as insulated gate bipolar transistors, IGBTs) connected in a bridge configuration with a plurality of phase legs. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor. The inverter may pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals applied to the gates of the IGBTs turn them on and off as necessary so that the resulting current matches a desired current.
Semiconductor switching devices such as an IGBT or a MOSFET are driven at a gate terminal by a gate signal provided by a driver circuit. For an IGBT, the gate signal is applied between the gate terminal and an emitter terminal of the device. In the ON state, an output signal is conducted through the device between a collector terminal and the emitter terminal. Device current flows in a gate loop and in a power loop.
Common source inductance refers to an inductive coupling between the power loop and the gate loop. Current in the output (power loop) portion of the common source inductance modifies the gate voltage in a manner that reinforces (e.g., speeds up) the switching performance. As disclosed in co-pending U.S. application Ser. No. 15/341,184, entitled “Inverter Switching Devices with Common Source Inductance Layout to Avoid Shoot-Through,” filed Nov. 2, 2016, and hereby incorporated by reference, the reduced switching time may be desirable since it may have an associated reduction in the energy consumed (i.e., lost) during the switching transition. The magnitude of the gate loop inductance and/or the power loop inductance and the degree of mutual coupling between them can be manipulated (e.g., enhanced) by selecting an appropriate layout and/or including added overlapping coils in PCB traces forming conductive paths to the transistor gates or emitters in order to obtain a desired common source inductance.
The transistor switching devices and associated components (such as a reverse diode across each transistor) are often packaged in a power module. A typical configuration known as a transfer-molded power module (TPM) implements one or more inverter phases by encapsulating transistor dies, diodes, and electrical interconnects (e.g., a lead frame) in an overmolded plastic body. The power module may be attached to a heat spreader plate which is thermally conductive to remove heat generated by the transistors. Active cooling using a “cold plate” can be used instead of or in addition to a heat spreader in order to remove greater quantities of heat more quickly. The cold plate typically includes internal channels to circulate a cooling fluid.
A heat spreader or cold plate may usually include electrically conductive plates or surfaces which extend parallel to the plane of the gate loop and power loop. For example, a cold plate may be comprised of an aluminum shell defining internal passages for conducting a flow of coolant. It has been discovered that Eddy currents generated in these surfaces can produce magnetic fields that can hinder the attempt to enhance the common source inductance.